Abstract : The paper suggested a method for analysis of boundary layers encountering a sudden change in boundary conditions based on a transformation into a new coordinate system which, though not appropriate for the original boundary layer, is nevertheless essential to the description of the perturbation caused by the new boundary conditions. Specifically, an incompressible, laminar boundary layer of the Falkner-Skan type is assumed to be suddenly accelerated by a belt moving in the plane of the wall at a constant speed in the downstream direction. General equations for such a flow are solved far away from, as well as close to, the location at which the boundary layer first encounters the moving belt. The results have shown that not only can the separation of the boundary layer be thus shifted infinitely far downstream, but also that a sufficiently rapid movement of the belt can produce in short distance a substantial acceleration of the flow leading to over-all frictional thrust instead of drag. As one may have intuitively expected, this last effect becomes greater the larger the belt velocity and the smaller the momentum loss of the original layer. (Author)